null pointer exception line 51 and print to screen?

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I'm getting a null pointer exception on the join method with the addAll dogs line 51 and it wont print to screen why?

1. Inside the Solution class, create public static classes Cat and Dog.
2. Implement the createCats method, which should return a set with 4 cats.
3. Implement the createDogs method, which should return a set with 3 dogs.
4. Implement the join method, which should return a combined set of all the animals (all the cats and dogs).
5. Implement the removeCats method, which should remove from the set pets all cats in the set cats.
6. Implement the printPets method, which should display all the animals in the set pets.
Each animal should be on a new line.

Requirements:

The program should display text on the screen.
There should be public static classes Cat and Dog inside the Solution class.
The Solution class's createCats() method must return a Set containing 4 cats.
The Solution class's createDogs() method must return a Set containing 3 dogs.
The Solution class's join() method should return a combined set of all the animals (all the cats and dogs).
The removeCats() method should remove from the set pets all cats in the set cats.
The printPets() method should display all the animals in the set pets. Each animal should be on a new line.

package com.codegym.task.task08.task0820; import java.util.HashSet; import java.util.Iterator; import java.util.Set; /* Animal set */ public class Solution { public static void main(String[] args) { Set<Cat> cats = createCats(); Set<Dog> dogs = createDogs(); Set<Object> pets = join(cats, dogs); printPets(pets); removeCats(pets, cats); printPets(pets); } public static Set<Cat> createCats() { HashSet<Cat> result = new HashSet<Cat>(); //write your code here result.add(new Cat()); result.add(new Cat()); result.add(new Cat()); result.add(new Cat()); return result; } public static Set<Dog> createDogs() { //write your code here HashSet<Dog> dogHashSet = new HashSet<Dog>(); dogHashSet.add(new Dog()); dogHashSet.add(new Dog()); dogHashSet.add(new Dog()); return null; } public static Set<Object> join(Set<Cat> cats, Set<Dog> dogs) { //write your code here HashSet pets = new HashSet(); pets.addAll(cats); pets.addAll(dogs); return pets; } public static void removeCats(Set<Object> pets, Set<Cat> cats) { //write your code here for(Cat cat:cats){ pets.remove(cat); } } public static void printPets(Set<Object> pets) { //write your code here for (Object pet : pets) { System.out.println(pet); } } //write your code here public static class Cat{ Cat(){ } } public static class Dog{ Dog(){ } } }